Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries. Among these secondary batteries, lithium secondary batteries having high energy density and output voltage, long cycle life and low self-discharge ratio are commercially available and widely used.
Specifically, increased concern over environmental issues has brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as substitutes for vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a major cause of air pollution. Although nickel metal hydride-based secondary batteries have mostly been used as a power source of such EV and/or HEV, a great deal of studies into use of lithium secondary batteries having high energy density and high discharge voltage is now extensively implemented and some of these are commercially available.
In conventional lithium secondary batteries, a carbon material is usually used as an anode active material and use of lithium metal, sulfur compounds and the like is also considered. Meanwhile, lithium cobalt oxide (LiCoO2) is most commonly used as a cathode active material and, in addition, other lithium transition metal oxides including, e.g., lithium manganese oxides such as LiMnO2 having a layered structure, LiMn2O4 having a spinel structure, etc., lithium nickel oxides such as LiNiO2, are also used.
Among the foregoing cathode active materials, LiCoO2 having excellent cycle life properties and charge-discharge efficiency is the most commonly used material. However, the above materials entail problems such as low structural stability and high costs for cobalt used as a raw material due to limited availability of cobalt resources, in turn reducing price competitiveness. Accordingly, there are restrictions on use of cobalt in large quantities in EV applications.
Meanwhile, although LiNiO2 based cathode active materials are relatively cheap while embodying cell properties such as high discharge capacity, they exhibit rapid phase transition in a crystal structure depending upon capacity variation accompanied by charge-discharge cycle and, when exposed to air and/or moisture, encounter sharp reduction in safety.
Lithium manganese oxides such as LiMnO2, LiMn2O4, etc. have merits of excellent thermal safety and low price but entail disadvantages such as low capacity, poor cycle life properties, poor properties at high temperature, etc.
Among these, spinel LiMn2O4 shows relatively uniform potential in the 4V region (3.7 to 4.3V) and the 3V region (2.7 to 3.1V). However, it is known that cycle life properties and storage properties of the above oxide are significantly deteriorated in the 3V region, thus causing difficulty in use thereof. The cause of this fact is that the above oxide is present in a single cubic phase in the 4V region due to phase transition based on Jahn-Teller distortion, while being converted into two-phase comprising the cubic phase and the tetragonal phase in the 3V region, and/or is dissolved into a manganese electrolyte.
For such reasons, when a spinel lithium manganese oxide is utilized in the 3V region, real capacity of the oxide is generally lower than a theoretical capacity of the same and C-rate properties are relatively low.
Therefore, it is known that utilization of spinel lithium manganese oxides in the 3V region becomes very difficult and few studies have focused thereupon, as compared to research and development into utilization of the same in the 4V region. Some studies have reported that cycle life properties may be improved by formation of a tetragonal phase or S-doping. However, such improvement is insignificant and/or exact reasons thereof have not been investigated.
Regarding utilization of lithium manganese oxides in the 3V region, Kang and Goodenough, et al. (Sun-Ho Kang, John B. Goodenough, et al., Chem. Mater. 2001, 13, 1758-1764) have proposed a technique for enhancing cycle life properties in the 3V region by forming nanograins and generating strain in the lithium manganese oxide by mixing a spinel lithium manganese oxide with carbon through milling. However, this method attains insignificant effects and does not explain reasonable grounds for improvement of cycle life properties.
The present inventors have found that conventional methods in the prior art including results of the foregoing studies may not embody desired charge-discharge properties in the 3V region.